Clay Plasters

Philip Allen and Neil May

Clay plasters
have been used extensively in buildings in the UK and indeed all
over the world, for thousands of years. Although it is not widely
known, there are probably over a million buildings with clay materials
in their structure in the UK, and a great many of these have clay
plasters. Very often clay plasters are not recognised because
they are painted or have a thin lime putty skim coat over them,
both internally and externally. Many clay plasters are still performing
well after many centuries, both in vernacular buildings and in
higher status properties (including their use in mouldings, ornamental
shields and so forth). The point of this article is to demonstrate
not only that clay plasters are important historically, deserving
proper repair and preservation, but also that clay plasters are
a viable and high quality material for use in standard conservation
and renovation work, the extension of historic buildings, and
even within modern building contexts such as for new buildings
in conservation areas and elsewhere.

Claytec
range of plasters, blocks, boards and ancillaries

WHAT
ARE CLAY PLASTERS AND HOW DO THEY COMPARE WITH OTHER PLASTER TYPES?

Clay undercoat plaster and, below, clay topcoat plaster

'Clay' refers
to the binder in the plaster, just as lime, cement or gypsum refers
to the binder in their respective plasters. Clay is therefore
a better description than 'earth', as earth also contains aggregates
and other materials which may be found in all types of plaster.
The clay used in buildings is generally a graded mixture of particles
in the clay and silt size ranges, which exhibit cohesion and plasticity.
The clay-sized particles can be either finely ground rock or clay
minerals.

The presence
of the clay minerals exerts a considerable influence on the properties
of the material, usually out of all proportion to the percentage
content. There are various types of clay mineral formed from combinations
of stacked crystalline sheets of silica and alumina. The most
common minerals are kaolinite, illite, and montmorillonite. These
have very different qualities due to the different arrangement
of the stacked sheets and the different strength of the bond between
the sheets. Basically, kaolinite has the strongest bonds (of hydrogen)
and forms the most stable, least shrinkable clay, but it has weaker
binding qualities and it is less hygroscopic (see below). Montmorillonite
forms weaker bonds between its sheets (which are linked by water
molecules as opposed to hydrogen molecules). Montmorillonite clays
therefore have better binding qualities, exhibit considerable
shrinkage and swelling and can absorb more water both through
capillary and hygroscopic actions. The characteristics of illite
clays fall somewhere between these two.

It is important
to note that the clay is unfired, so it is not the same as the
fired clay particles added to some lime plasters as pozzolans
to make them set hydraulically. Fired clays have very different
qualities.

Clay plasters
(and mortars) also distinguish themselves from other plasters
(and mortars) by the way that they bind and then harden or cure.
For cement, gypsum and to some extent hydraulic limes, the process
of curing is through a hydraulic reaction between the binders
and water. For non-hydraulic limes, and to a lesser degree for
hydraulic limes and cement, the process of curing is through carbonation,
the conversion of calcium hydroxide to calcium carbonate by the
absorption of carbon dioxide from the atmosphere. Clay plasters,
however, become solid as excess water added during mixing is lost
and the electrically charged surfaces of the clay particles move
closer together. The relative strength of the forces of attraction
and repulsion between clay particles is responsible for the amount
of cohesion and binding force of clay in the plastic state and
for the compressive and tensile strength in the solid state. The
strength of the attraction between the clay particles can increase
if the particles are brought closer together by external pressure
(usually by a trowel or float, or if thrown) or if the water surrounding
the particles has a higher concentration of positive ions (for
example by the addition of additives such as urine). In addition
to these electrostatic charges there are also capillary, electromagnetic,
frictional and cementitious forces at a molecular level.

WHAT
ARE THE QUALITIES OF CLAY PLASTERS COMPARED TO OTHER PLASTERS?

Clay plasters
have very specific and, in many ways, unique qualities which are
extremely well suited to historic building conditions for a number
of reasons. These may be dealt with under the headings: breatheability, flexibility,
reversibility and aesthetic qualities.

Breatheability
In terms of their ability to 'breathe', clay plasters not only
have excellent vapour permeability (a factor of around eight
- that is to say, only eight times the equivalent thickness of
air), but also extremely good hygroscopic qualities. What is significant
here is not only the amount of moisture that can be absorbed from
the air but also the rate of absorption. In most materials hygroscopic
qualities relate to the capillary structure of the material, whereas
in clay plasters, moisture can also be drawn in and held by ionic
bonding with the clay particles themselves. For this reason the
type of clay is highly significant, as explained above. Kaolinite
clays have less hygroscopic qualities, while montmorillonite clays
have the ability to take in moisture very rapidly from the atmosphere
when humidity rises. On the other hand, burnt clay and expanded
clay have very poor hygroscopic qualities.

Clay
plasters in a historic timber frame house

Because the
rate at which clay plasters absorb moisture is much higher than
that of other materials (timber, for example, takes in and releases
large quantities of moisture but over a much longer period), clay
plasters can act to protect vulnerable organic materials (and
in particular timber) from high levels of relative humidity, when
microbial and insect attack can be triggered. This can be an important
strategy in the control of excess moisture in vulnerable buildings,
threatened by increases in moisture level caused by showers, general
indoor living, draught-proofing measures and the switch from open
fires to central heating in particular. In addition to the building
benefits of clay, the hygroscopic qualities mean that moulds caused
by condensation are minimised, and that a relative humidity of
50 to 60 per cent is maintained. This is the ideal level for mucous
membranes of the human body, and also for the control of dust
mites and other organisms which affect human health.

Clay plasters
also have very good capillary qualities. They actually have less
capillary draw than materials like lightweight brick, and even
certain cement products, but more capillary draw than most types
of timber. This means that within an exposed traditional timber
frame building they will draw water droplets away from the timbers.
By comparison, hard renders (including those containing eminently
hydraulic lime or a high proportion of Portland cement) are notorious
for drawing in moisture at the crack where it abuts the timber
and holding it there, causing the timber to decay.

Flexibility
Clay
plasters are flexible in relation to their fibre content. In this
sense they are similar to fat lime plasters. Their inherent soft
and pliable qualities mean that fibres such as straw, flax, and
hair are able to hold together the plaster without cracking in
situations of minor or gradual movement, provided there is sufficient
quantity of fibre. This is a significant quality in old buildings.

Reversibility
Clay plasters, to an even greater extent than fat lime plasters,
are reversible. Unlike lime, cement and gypsum, clay is also reworkable,
provided it is not contaminated (particularly by salts). Clay
plasters are not only easy to remove, but they do not have the
staining or caustic qualities of lime. Because of their 'breathing'
qualities they can protect more vulnerable parts of structures,
and absorb large amounts of moisture, salts and pollutants where
these are a danger - in other words, they can be used sacrificially.

Aesthetic
Unpainted clay plasters have a very particular aesthetic. Due
to the shrinkage of the clay particles on drying, the plasters
always have an open texture even when polished. This means that
light reflects and refracts on the surface in such a way that there
is always variation and never a gloss sheen. This is particularly
noticeable in the modern self-coloured plasters that are becoming
more commonly available.

CLAY
PLASTERS IN PRACTICE

Earth materials
were used extensively in historic buildings of all sorts prior
to the 19th century. Their disappearance was partly an effect
of the separation of agriculture from construction as a result
of the industrial revolution. The disappearance was not because
the material was considered primitive or inadequate in any way,
but because of the changing economic conditions of the time. This
is an important fact to remember when looking at clay plasters
in practice. These are not primitive materials, even if they are
not synthetic or a result of high energy processes. Similarly
the raw ingredients are not simple, even if they are commonly
available.

Clay plasters
have many complex qualities and require a proper understanding.
Given that understanding, they can perform very effectively, and
are durable and attractive.

Preparation
Clay plasters can either be made from local soil, or can be bought
in a proprietary form. If in a proprietary form, they come dry
bagged and are mixed with water to a plastering consistency, prior
to application. If the plasters are being made on site or from
local material, a number of issues must be addressed, such as
the type of clay, the size of aggregate, the proportion of clay,
the even dispersion of clay (a problem if the clay is wet), the
addition of fibres and many others. If this approach is to be
taken, then soil analysis, good crushing and grinding machinery,
possibly drying machinery and a lot of patience and knowledge
are required. All clay plasters can be stored dry indefinitely.
If already mixed up with water the only material that will go
off will be the fibre, and this will take several months.

Application
Clay plasters may be applied like any other plaster, by hand or
by spray application. As with other plasters, background suction
must be carefully controlled by wetting it, but without saturating
it, and for top coat application the suction should be even across
the surface. A very important point to note is that clay plasters
should not be over-wetted during the plastering process. Too much
water will increase the shrinkage on drying and then the plaster
will crack or powder when it dries: lightly wetting just prior
to application and then working wet on wet is the best strategy.

A
15th century mixed timber frame and stone house in Garsington,
Oxon, built originally with clay mortars and plasters throughout
and renovated by Neil May Builders in 1999, using entirely
clay undercoat and topcoat plasters.

Clay plasters
cure by the evaporation of water. If there is insufficient background
suction, heat or air movement then they will not dry, but will
stay soft. After some time moulds may also develop, particularly
if there are natural fibres in the plasters. This is natural and
can be reversed by carefully controlling the temperature and the
amount of ventilation to improve the drying conditions. However,
if there is too much suction, heat or air movement the plasters
can dry too quickly and this can cause a separation from the substrate,
and powdering of the surface. If this occurs then the product
can be reworked as appropriate, either on the wall, or by being
removed, knocked up again with water and reapplied.

If a mesh
is required then hessian or glass fibre is best. Metal beading
(galvanised or stainless) can also be used, but the heavy metal
beading common for gypsum plastering is not suitable. On angle
beads in particular a thin wire bead should be used and the clay
plaster should cover the corner bead entirely. For 'tight', less
open grained plaster finishes a fine slightly damp sponge is used
to consolidate the clay. For polished finishes specialist plastic
trowels are used. If a steel trowel is used at this stage then
the clay can be disfigured.

Limitations
and protection requirements
Although clay plasters and renders are more vulnerable than lime
renders if incorrectly detailed and protected, they can be used
in all locations internally and externally without protection
except where they are subject to direct spray. When used externally
they do need a limewash or a fine lime plaster skim to protect
them against driving rain, depending on levels of exposure. Rain
resistance can also be substantially improved by controlling the
proportion of clays and fibres in the mix, and by additives such
as casein.

Where exposed
to direct spray internally (for example in showers, just above
sinks and in other vulnerable areas), it is necessary to use tiling
or some other impermeable material. It is also suggested that
timber is preferable to clay plaster for lining the reveals around
cold windows where condensation occurs frequently on the frames.
Where clay plasters get wet only occasionally, then a coat of
casein or silicate paint, or limewash is usually sufficient protection.

Clay plasters
can also be coated with matt emulsions and other more conventional
paints, so long as these paints are reasonably vapour permeable
(a factor of no more than 300). However, a build up of successive
coats of emulsion will reduce their permeability, and rubbing
down between coats can abrade the surface of the plaster. Non-vapour
permeable coatings and harder coatings will cause problems in
the long term for the plasters, and obviously affect their breathing
qualities. Clay plasters have good impact resistance, equivalent
to that of many lime plasters, but they are not robust enough
for all situations. Where a wall is subject to hard use or frequent
impact (such as in an entrance lobby or at floor level for example)
skirtings, dado rails, or panelling should be used.

Clay plasters
are often overlooked or even purposely ignored in the repair and
conservation of historic buildings. The reasons for this are partly
a lack of understanding about the technical qualities of clay
and partly a lack of available (clay) materials for repair. These reasons should no longer be regarded as valid, however. It is vital for both the technical performance
and the integrity and character of historic buildings that clay
plasters and renders are respected, preserved appropriately and
repaired where necessary with authentic (like for like) materials.
It is to be hoped that, in the not-too-distant future, clay technology
will, once again, become a major part of the preservation of our
historic building heritage.

This
article is reproduced from The Building Conservation Directory, 2003

Author

PHILIP
ALLEN has a degree in civil engineering and is a corporate
member of the Institution of Structural Engineers. He works
as a freelance engineer and as a long-term consultant to
NBT (Natural Building Technologies). He worked for six months in Tanzania on earth construction,
and has also worked on the construction of several
earth buildings in the UK.

NEIL
MAY studied modern history at Oxford University and then
an MPhil in sociology at Delhi University, India. During
this time he co-directed an award-winning documentary film
about the coalfields in Bihar. In 1994 he founded Neil May
Builders, an award winning ecological and conservation
building company. He founded NBT, a company
that develops and sells ecological building materials, in 1999.